Hello,
Yesterday I was at a friend's house,he has a VT100mkII.
Anode resistor R44 of 1ohm died. Put in a new one,and this tube actually works fine,but the V12 has too much bias.(there are some traces of voltage-sparking [?] near V12) Measuring the inputs bias without all 6550, does not give good results. Shortly after Q1 died and now I need to replace it.
Have any of you come to a conclusion.
Does that DN2530 really work; should I order DN2535 also, just in case?
Hope we will get these amps working again and find the right Fet for all the Audio Research amps...[emoji16]
Regards, audiomania
Yesterday I was at a friend's house,he has a VT100mkII.
Anode resistor R44 of 1ohm died. Put in a new one,and this tube actually works fine,but the V12 has too much bias.(there are some traces of voltage-sparking [?] near V12) Measuring the inputs bias without all 6550, does not give good results. Shortly after Q1 died and now I need to replace it.
Have any of you come to a conclusion.
Does that DN2530 really work; should I order DN2535 also, just in case?
Hope we will get these amps working again and find the right Fet for all the Audio Research amps...[emoji16]
Regards, audiomania
Attachments
I couldn't get it to work with the DN253 nor will ARC send /sell you the parts.The MOSFET's in the VT200 are a little different.I don't think there is much difference though.
BluesTM got his amp to work.I broke down and had a local tube amp specialist look it over. Cultrit still seems to be the mosfets.Give it a shot but don't put the 6550's in until your bias voltages are correct or your toast a bank of 6550's.
Thx audiodoc for your quick reply.
Reading a lot and trying to understand. The Q1-2 are just normal CCS with 3,4mA as I see it.
It shouldn't be a problem to adjust a depletion mode MOSFET to give 3,4mA on around 36V
I have to get into depletion mode more to understand and find a solution.
After getting the frontend working good again, I will have to look at why R44 got shot and even the mainsfuse got blown. Looks like a problem there also.
Audiodoc, did your local tube specialist fix it and how?
Regards, audiomania
Reading a lot and trying to understand. The Q1-2 are just normal CCS with 3,4mA as I see it.
It shouldn't be a problem to adjust a depletion mode MOSFET to give 3,4mA on around 36V
I have to get into depletion mode more to understand and find a solution.
After getting the frontend working good again, I will have to look at why R44 got shot and even the mainsfuse got blown. Looks like a problem there also.
Audiodoc, did your local tube specialist fix it and how?
Regards, audiomania
I have some feedback on the broken VT100 MKII.
My friend gave it to Audioservice in NL.
They changed 4 of these fet things, they ordered at ARC. They also changed all 6550 to new ones.
We tried to get these fet from ARC directly, but they did not sell it to us.
After repair, the VT100 MKII sound very very good! But actually, because you cannot repair these things yourself, I cannot recommend ARC to anyone...
The repair was done at a reasonable price btw!
My friend gave it to Audioservice in NL.
They changed 4 of these fet things, they ordered at ARC. They also changed all 6550 to new ones.
We tried to get these fet from ARC directly, but they did not sell it to us.
After repair, the VT100 MKII sound very very good! But actually, because you cannot repair these things yourself, I cannot recommend ARC to anyone...
The repair was done at a reasonable price btw!
my stomach is in a knot
I am looking at replacing 3 dead Fets in one channel, so this whole process is interesting to me... I was / have been agonizing over the considerations for ARC service .not. possibly selling me them. I have a few ideas, a few contacts that me be able to help.
I used the DCA-75 as well, very nice little device... I trust it!
I recapped my VT200 (it needed it) a few caps had distorted plastic wrap, suggesting prospects of overheating and the notable buzz in that channel. ARC want / offered to sell me the original (20 year old) caps, which I would consider like NOS at this point, not really trust them... I declined.
I want to throw this out there - apologize, not wanting to hi-jack this thread... but I will consider it related to [safely] operation of VT100/200 amps with like biasing set-up. I am incline to install B+ fuses in place of? the 1 ohm plate resister... I want to limit unmentionable damage in the event of output tube failure. I would rather "risk" a few percent performance = in favor of safety a limiting damage... Note: my amps has never blown output tubes, during my ownership or (by appearances prior) the boards and traces are very clean.
Thanks for pointing out the bi-polar transistor option. I am incline to try it, especially given the nature of the use in the circuit. Sure using FETs sounds good has solid bragging rights... I am all for reliability and stability, regarless of the steps.
**the stomach in a knot comment - I really don't like working on high voltage gear... Been building SS amps for forty years, don't mind them as much... I refused to send it to ARC and spend 1,000~ maybe waaaay more.
I am looking at replacing 3 dead Fets in one channel, so this whole process is interesting to me... I was / have been agonizing over the considerations for ARC service .not. possibly selling me them. I have a few ideas, a few contacts that me be able to help.
I used the DCA-75 as well, very nice little device... I trust it!
I recapped my VT200 (it needed it) a few caps had distorted plastic wrap, suggesting prospects of overheating and the notable buzz in that channel. ARC want / offered to sell me the original (20 year old) caps, which I would consider like NOS at this point, not really trust them... I declined.
I want to throw this out there - apologize, not wanting to hi-jack this thread... but I will consider it related to [safely] operation of VT100/200 amps with like biasing set-up. I am incline to install B+ fuses in place of? the 1 ohm plate resister... I want to limit unmentionable damage in the event of output tube failure. I would rather "risk" a few percent performance = in favor of safety a limiting damage... Note: my amps has never blown output tubes, during my ownership or (by appearances prior) the boards and traces are very clean.
Thanks for pointing out the bi-polar transistor option. I am incline to try it, especially given the nature of the use in the circuit. Sure using FETs sounds good has solid bragging rights... I am all for reliability and stability, regarless of the steps.
**the stomach in a knot comment - I really don't like working on high voltage gear... Been building SS amps for forty years, don't mind them as much... I refused to send it to ARC and spend 1,000~ maybe waaaay more.